The midfielder was missing from the Pompey XI squad which inflicted an 8-0 hammering on Newport IW last night.
Nicke Kabamba bagged five goals, while Jamal Lowe (two) and Milan Lalkovic also wrote their names on the scoresheet as the Blues ran riot at St George’s Park.
Roberts had already been sidelined for Saturday’s victory at Bognor through a back problem.
With Kenny Jackett seeking to offload attacking players, his continued lack of involvement prompted some to question whether the former Chesterfield player could be leaving Fratton Park.
But Gallen explained that while the 33-year-old has now recovered from his back niggle – he left it too late to declare himself fit for the trip to Newport.
‘Gary pulled out of training on Friday during the warm-up,’ said Gallen.
‘I was taking it so maybe it wasn’t a very good warm-up by me – but he just felt his back and went inside.
‘Sometimes the ground is hard and he did a lot of running the previous Tuesday night at Poole Town.
‘We didn’t even realise he was going to be fit yesterday morning, but he declared himself available for Newport, which was a bit too late.
‘We had done the plan, had our pattern, put together set-pieces and had the organisation when Gary wasn’t fit the previous day.
‘He’s now fit, though, and he will be up for selection on Saturday against Bournemouth.’
Meanwhile, Gallen revealed Michael Smith is now available for selection following his ankle problem.
He added: ‘Michael is fit and training. He’s up for selection and it’s up to him to push on and get in.’
– NEIL ALLEN